Continuous ink-jet printers have been in commercial use for many years for labeling. According to the operating principle of these ink-jet printers, ink is pumped from a reservoir to a pressure chamber located in the actual print head. This pressure chamber has a gun or nozzle on the side facing the material to be printed. The nozzle may have an opening diameter of 30 μm to 200 μm. The ink is initially emitted from the nozzle in the form of a continuous ink stream, which, however, is not practical for labeling, since the print characters produced in this type of labeling are composed of individual dots created by individual ink droplets. To disperse the ink jet into individual ink droplets that are uniform and in particular of the same size, a modulation element is mounted on the pressure chamber that produces pressure fluctuations in the ink jet emitted from the print head, so that after exiting the nozzle, in particular after a short time and at a defined distance, the ink stream breaks up into individual, and in fact uniform, ink droplets.
The size of the ink droplets depends, among other factors, on the applied modulation frequency, the nozzle diameter, the ink's surface tension, and the pressure produced by the pump, and may be adjusted within the system limits specified by the combination of these parameters. Therefore, it is not possible to vary the droplet size of successive ink droplets to any significant extent.
Shortly before the ink droplets separate from the emitted ink jet, each of the ink droplets is given a predetermined electrical charge whose magnitude depends on the desired impact position on the product to be labeled. The ink has a low electrically conductivity to hold the electrical charge. During the charging process the ink droplet has not yet separated from the ink stream emitted from the nozzle of the ink-jet printer, so that as a result of the electrical influence, free charge carriers in the ink are moved either toward or away from the charging electrode, depending on the polarity and intensity of an external charging voltage, and the ink chamber and thus the ink reservoir, for example, are electrically held at ground potential. The charging electrode has no mechanical contact with the ink stream.
When the ink droplet separates from the ink jet while it is in the field region of the charging electrode, the influenced electrical charges that have migrated into the droplets remain in the droplet that has an external electrical charge, even after the separation. If the charging electrode is positively charged, for example, when the ink jet enters the electrical field of the charging electrode the negative free charge carriers in the ink migrate into the field, and the positively charged free charge carriers in the ink are ejected from the electrical field. A charge separation thus occurs at the front edge of the ink stream, immediately before the droplet separates, and the charge imbalance thus produced is maintained in the separating droplet, and the droplet, which in this example is negatively charged, leaves the field region of the charging electrode. Since the ink stream separates into droplets as the result of the design and operating principle, a charge remains on the separated ink droplet as described, whose magnitude corresponds to the value of the applied charging voltage at a constant electrical conductivity of the ink, so that when the charging voltage changes, the charge level may also be changed on each droplet.
On their initial linear trajectory, the electrically charged ink droplets pass into the electrostatic field of a deflecting device such as a plate capacitor, and, depending on their individual charge, are deflected to a greater or lesser degree from their linear trajectory, and after leaving the electrostatic field continue traveling at a given angle relative to their original trajectory which is a function of their charge. They eventually hit the substrate or target at a location determined by how much they were deflected, and, if the are not deflected at all, they are intercepted by a gutter and recycled back to the ink supply.
According to this system, it is possible to select different impact positions on a surface to be labeled with individual ink droplets. Normally this occurs in only one deflection direction. To eliminate individual droplets from the print image or if printing is not to be performed, as described above the ink droplets are provided with a specified fixed charge or remain uncharged, so that after emerging from the electrostatic field of the plate capacitor they strike a collection tube and are pumped back to the ink reservoir. The unprinted ink thus circulates in a circuit, adding further meaning to the term “continuous ink-jet printer.”
A disadvantage of the described design is that, due to the system-related production of the ink droplets, these ink droplets always have the same size within narrow tolerances, so that a print image produced with these droplets always has the same size print dots.
In contrast, it is known from printing technology that for producing grayscales and color gradients in printed images, different sizes of print dots are used to give the observer a visual impression of grayscales or color gradients. Thus, for example, for all printing methods that use a printing plate, the individual print dots are designed with different sizes according to a pattern in production of the printing plate, resulting in different sizes of print dots in the printing.
For drop-on-demand (DOD) ink-jet printers, it is also known to use different sizes of print dots. This is achieved by the fact that during printing a differing number of uniformly sized small droplets is superimposed on the surface of the material to be printed to produce an overall larger print dot.
A disadvantage of the known methods is that they do not permit the print data to be varied within the printing process, since they operate in conjunction with printing plates, or, in the case of the DOD methods, as a result of the system and in particular in the labeling region there is only one disadvantageously small working distance of the print heads from the surface to be printed. In addition, since DOD printers always have multiple nozzles in a print head, only inks that are non-drying or slow-drying inks, or radiation-curing inks, may be used, since otherwise the inks in individual nozzles that receive little or no use dry out, causing these nozzles to fail.
Although the use of radiation-curing inks eliminates this problem, the additional use of subsequent curing appliances entails significantly greater complexity of equipment and higher costs. In addition, as a result of the typically small operating distance it is not possible to label, for example, a textured surface with high print quality, since after a short distance the trajectories of the emitted ink droplets become so unstable that a desired impact position cannot be reliably achieved, and therefore a print dot composed of multiple ink droplets can no longer be printed as a closed print dot having a defined shape.
In these systems, use of the above-mentioned inks is mandatory, since such DOD systems operate with numerous individual nozzles that may be actuated as needed. It is normal for an individual nozzle to have little or no actuation for a fairly long period of time, depending on the print image to be printed, so that when a quick-drying ink such as a solvent-containing ink is used, the ink in this nozzle dries out and the nozzle opening becomes plugged.
If this nozzle is required at a later time it is no longer available, necessitating cleaning, frequently manually, of the print head. In contrast, continuous ink-jet printers are able to print using inks that have an extremely short drying time, since the solvents used in these inks evaporate very quickly.
Since in this type of ink-jet printer the ink is continuously emitted from the nozzle, it is not possible for the nozzle to become plugged and the process to be interrupted. However, with continuous ink-jet printers of this type it has not been possible heretofore to selectively produce different sizes of ink droplets within a print image.